sometimes when I left the car running idle for 5-10 minutes, there is a clicking/ticking noise comes from the engine (sounds like valve noise), but after I use the car for sometimes, the clicking/ticking noise dissapeared.

any comment?

 

Most likely the oil pan O-rings.

 

tech,

When you get a chance can you please explain that one? I don't get it, oil pan o-rings? What are they and how would they make a noise?

Not that I'm doubting your advice, it's always right on.

 

The oil pan o-rings go between the oil pan and the engine. There is oil ports that go through the oil pan and the engine. When the o-rings go bad oikl pressure is lost and causes a tapping noise from the top of the engine.
check out this post for replacing them and where they are.
https://volvoforums.com/m_6169/tm.htm